Amongst other tasks helicopters are employed for load-carrying flights, wherein an item of freight, mounted outside the helicopter cabin, is transported by the helicopter from A to B. Various materials for various construction or maintenance tasks can be transported as the load, or in the case of rescue flights, a stretcher with living persons. As a general rule a load hook that can be remotely operated, with or without a load cable, is arranged on the outside of the helicopter, directly on the underside of its cabin, wherein during the load-carrying flight tensile forces are transferred directly from the load hook onto the underside of the cabin. Such a design is a simple and cost-effective solution, wherein the load hook can easily be removed when it is not in use. The load hook, or more particularly, a load hook substructure to which the load hook is attached, can participate in small swinging movements of the load during the flight; the possibilities for movement are however limited by the design, and such a structure is rather rigid.
Other forms of embodiment are of known art, in which the load hook is attached to a separate load hook substructure. Here the load hook substructure is arranged such that it projects from the helicopter cabin in the direction of the skids, and thus is spaced apart from the underside of the helicopter cabin. The load hook on such a load hook substructure can accordingly be swung further, as allowed by the design, and such a system has a small degree of flexibility. The load hook substructure in the form of a mounting frame is bolted onto attachment points on the underside of the cabin, and can similarly be removed when not in use, together with the load hook. However, such mounting frames are bulky and have inferior aerodynamic properties. In addition to the higher costs of such a mounting frame the higher weight and maintenance tasks required by the mechanical design are disadvantageous.
During a load-carrying flight high tensile forces act in different ways on the load hook, and indirectly on the underside of the cabin. A load is usually attached to the helicopter in hovering flight, as shown in FIG. 1A. The load rests on the ground and while the helicopter slowly rises, the load cable becomes taut by virtue of the tensile force of the load. As shown in FIG. 1B a maximum tensile force acts on the load hook, and thus on the helicopter cabin, at the point at which the load lifts from the ground. The load hook, load cable, load hook substructure, and helicopter cabin must be able to withstand these peak loads. During forward flight (FIG. 1C), or during flight manoeuvres of all kinds with an accompanying load, vibrations occur, wherein the load vibrates at various frequencies in the direction of gravity.
The load hook substructures of load hooks used up to the present time, together with the load hooks themselves, have no precautionary measures, or virtually no such measures, for reducing the peak loads, or for damping any vibrations that may occur. Since the flying characteristics of the helicopter are affected by the high tensile forces of the loads, and also by the vibrations, the peak loads and vibrations should be kept as low as possible.